Last month’s deadly sinking of a South Korean naval ship was caused by a North Korean torpedo, a news report said today, adding to pressure on the South’s President. Lee Myung Bak, to respond to one of the worst acts of military provocation since the Korean War.

The South Korean defence ministry declined to comment on the claim by the Yonhap news agency, the latest in a series of reports suggesting that the mysterious sinking of the naval corvette, Cheonan, on March 26 was a deliberate and unprovoked attack by North Korea.

Forty-six sailors are dead or missing after the attack, which cut the 1200-tonne vessel in two. But President Lee’s Government appears to be struggling to find an appropriate response, which will demonstrate its resolve in the face of aggression, but stop short of a costly and unpredictable war.

“Military intelligence made the report to the Blue House [the presidential palace] and defence ministry immediately after the sinking of the Cheonan that it is clearly the work of North Korea’s military,” an unnamed military source was quoted as telling Yonhap. “North Korean submarines are all armed with heavy torpedoes with 200kg warheads. It is the military intelligence’s assessment that the North attacked with a heavy torpedo.

In a sign of the continuing confusion surrounding the incident, a separate report suggested that the attack was caused not by a remotely fired torpedo but by a manned suicide submarine which exploded after being piloted under the ship’s hull. The Chosun Ilbo newspaper suggested that this was an act of retaliation for a naval skirmish last November in which a handful of North Korean sailors are believed to have been killed and a patrol boat severely damaged after an exchange of fire with South Korea’s more modern navy.

“Military authorities detected several signs showing that the North was preparing for revenge for its defeat in the sea skirmish in November last year,” an unnamed government official was quoted as telling the Chosun Ilbo newspaper. “The North intensively trained military units for various means of attack, in particular human torpedoes.”

President Lee’s conservative Government prides itself on taking a tough attitude to North Korean aggression, by comparison with its liberal predecessors who sometimes played down provocations in the interests of good long-term relations with Pyongyang.

“If they fire two bullets at us, we will fire three or four back,” a government official told The Times last year. “If they fire on us from a shore battery, we will take it out.”

But a month after the sinking of the Cheonan, Mr Lee appears to be struggling to find a response which will both satisfy public opinion and preserve peace. The South’s superior equipment makes it quite capable of launching a retaliatory strike. But such a response could intensify unpredictably into a full-scale war, which might result in eventual victory for the South and its US allies, but could be ruinously destructive and expensive.

In some ways, a limited war might be exactly what the North Korean leader, Kim Jong Il, is hoping for. After decades of economic decline and famine in the 1990s, which killed as many as a few million people, his economy is in chronic decline. A military adventure, against the routinely demonised “imperialist” US and its South Korean “lackeys” could serve as a welcome and unifying distraction.

“It’s obvious that Kim Jong Il did it,” said Hwang Jang Yop, a former senior North Korean politician who defected in 1997, in an interview published today. “We already know Kim Jong Il has been preparing for this kind of incident ... [But] if we retaliate, the peninsula will turn into a dirty war zone like the Palestinian territories. We must not be drawn into their scheme.”

Part of the problem is that the South has failed so far to find any clinching evidence, such as fragments of an irrefutably North Korean torpedo, to prove what it suspects. Today, naval engineers failed once again to attach lifting chains to the front half of the Cheonan, which still lies on the seabed and which may contain the necessary proof.

Meanwhile, South Korean public opinion has been dismayed by what many see as irresolute handling of the incident and a reluctance to tell the truth to relatives of its young victims. “No one wants to say it out loud,” wrote Song Ho Keun, a professor at Seoul National University in the Joong-Ang Ilbo newspaper. “We told ourselves to be patient and cool, not to jump to conclusions as there is no definitive evidence implicating the North. But if we find one little piece of evidence pointing definitely at North Korea, the rage we have forcibly suppressed will gush forth.”